As human babies become more familiar with the human face, they lose the ability to distinguish between different monkey faces.

This finding, recently reported by University child development professor Charles Nelson in the journal Science, could help doctors better understand conditions such as autism, in which sufferers fail to develop emotional attachments to familiar people.

Nelson, co-author of the study and co-director of the University Center for Neurobehavioral Development, was drawn to his research by an early interest in speech development studies. Those studies show that 6-month-old babies can distinguish between sounds in any language. As they have more experience with their native tongue, they lose the ability to distinguish between the sounds of other languages.

“It is the reverse of how we think about development,” Nelson said. “Usually when you gain experience, you get better at something.”

Nelson wanted to find out if the same is true for the development of facial recognition. He recruited 30 6-month-old and 30 9-month-old babies, who were each shown identical pictures of either a human or
monkey face for 30 seconds. They were then shown the same face with a new one and timed to see which picture they looked at the longest.

Many studies have shown that babies prefer the unfamiliar, and Nelson’s babies were no exception – they all spent more time looking at the novel face than the familiar one.

Nelson said all of the babies could distinguish between familiar and non-familiar human faces. Only the 6-month-olds, however, were adept at distinguishing primate pictures as well.

This means that the ability to
distinguish faces of other species is lost with age or experience, which suggests that experience shapes the way the brain develops, Nelson said. If this is true, facial recognition is not innate.

Nelson likens the development to learning to distinguish between different models of cars.

“We aren’t born with the ability to do it, just the ability to learn,” he said.

If the ability to discriminate between faces is learned, then autism and other behavioral disorders could be treated by training sufferers to pay more attention to faces, Nelson said.

“Autistic people look at mouths and eyes,” he said, “not at faces.”

People with autism and other behavioral disorders don’t recognize faces, Nelson said. This could occur either because the areas of the brain that discriminate between faces are impaired or because those areas were never trained to recognize faces.

He also believes that children who grow up with depressed mothers,
children who were abused and children born with cataracts have problems recognizing faces, he said.

Training them could improve the ability to recognize faces and the emotional meaning in their expressions.

Nelson said he hopes to learn more about that ability through another study, in which half of a group of 6-month-olds is shown a book of monkey faces every day for three months, while the other half is not. Those who see monkey faces every day, Nelson suspects, will be better able to distinguish between them than the babies who did not see them every day.

Nelson is planning a study to see if babies at different ages will respond to mock human heads the same as to their mothers. He wants to know if babies respond to their mothers’ faces because they are familiar or simply because they exhibit facial expressions.